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Population Trends and Management
opportunities for Neotropical Migrants
Chandler S. ~obbins',John R. ~auer',and Bruce G. peterjohn'
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Abstract
The Breeding Bird Survey shows that certain Neotropical
migrant songbird populations have been declining over the past 26 years.
Among them are forest birds that require extensive forest on the breeding
grounds and also forested habitats on tropical wintering grounds. Other
species have shown significant declines only since the early 1980's. Birds
with broader habitat tolerance, such as those that winter commonly in
agricultural and early-successional habitats as well as primary forest, show
fewer consistent declines. Several grassland species have also been
declining for more than two decades. Populations of many other Neotropical
migrants have been stable or increasing over these periods. Examples of
26-year population trends are given. A dozen recommendations are given
for managing nesting habitat for Neotropical migrants.
WHAT DO WE KNOW ABOUT
POPULATION TRENDS?
Certain species of neotropical migrant songbirds have been
decreasing in abundance throughout their breeding range for
prolonged periods and are a cause of immediate concern
Examples of continuing declines from 26 years of Breeding Bird
Swey (BBS) data are Olive-sided Flycatcher, Cerulean Warbler,
Kentucky Warbler, Lark Bunting, Dickcissel, Grasshopper
S p m w (fig. 1); Rock Wren, Prairie Warbler, Painted Bunting,
and Black-throated Sparrow (fig. 2). Marry other species not
declining in the 1960's and 1970's show strong declines during
1982-1991. Trend graphs are presented for two of these species,
Wood Thrush and Bobolink (fig. 2). Other examples are
Yellow-billed Cuckoo, Ruby-crowned Kinglet, Veery,
Chestnut-sided Warbler, Macgilhvray's Wahler, Rose-breasted
Grosbeak, and Savannah Sparrow. Many additional species are
decreasing in portions of their breeding range where their nesting
habitats have been disappearing rapidly. Examples are Lark
Sparrow in the west, Whiteeyed Vireo and Hooded Warbler in
the central states, Canada Warbler in Canada, and
Yellow-breasted Chat and Vesper Sparrow in the east. There are
also many neotropical migrants whose populations have been
increasing during the period 1966-1991. Examples are
Ash-throated Flycatcher, Western Kingbird, Solitary Vireo,
'
U.S. Fish and Wldlife Service, Patuxent Wildlife Research Centel;
Laurel, Maryland, U.S.A. 20708-4015.
Magnolia Warbler, Blue Grosbeak, and Lincoln's Spanow '(fig.
3). Continental populations of the majority of neotropical
migrants have remained stable or increased during the 26-year
period, but with regional population declines for many species.
Discussion of trends in species other than neotropical
migrants is beyond the scope of this paper, but important
negative trends have been noted for grassland and early
successional species at continental as well as regional scales
(Robbins et al. 1986, Hagan et al. 1992, Hussell et al. 1992,
James et al. 1992, Witham and Hunter 1992).
THE BREEDING BIRD SURVEY
Thlse latest population trends are from unpqblished BBS
analyses, BBS is a cooperative monitoring program administered
by the U.S. and Canadian Wildlife Services. This annual survey
consists of a network of randomly distributed roadside routes,
each with 50 3-minute stops at 0.8 krn (half mile) infervals along
seconday roads (Robbins et al. 1986). Participants are almost
entirely experienced volunteers, hand-selected by state and
provincial coordinators. At present more than 2,200 routes,
representing more than 110,000 point counts, are run each year.
The resulting records, for more than a million birds per year,
go through intensive editing procedures before being added to
the data bank. Sophistiated analysis programs compensate for
density of coverage and changing observers on a route (Geissler
and Sauer 1990, Sauer and Geissler 1990). Estimates of
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Year
Year
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Year
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Figure 1.
Examples of long-term continental declines in three woodland (A, B, C) and three grassland species (D, E, F). A Olive-sided
Flycatcher, B Cerulean Warbler, C Kentucky Warbler; D Lark Bunting, E Dickcissel, F Grasshopper Sparrow.
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Figure 2.
Examples of long-term continental declines in four brushland and early second-growth species (A, B, C, D),'representing
four different portions of the continent, and short-term declines of a woodland (E) and a grassland species (F) whose populations
were stable until the 1980's. A Rock Wren, B Prairie Warbler, C Painted Bunting, D Black-throated Sparrow; E -Wood Thrush,
F Bobolink.
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Figure 3.-Examples of long-term continental increases in six species of forest, field, or brush-nesting birds, representing all parts of
North America south of the tundra. A Ash-throated Flycatcher, B Western Kingbird, C -Solitary Vireo, D Magnolia Warbler, E
Blue Grosbeak, F Lincoln's Sparrow.
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4
population change, together with statistical probabilities, are
geE& by species for each state, province, physiographic region,
mjor region of the continent, and the entire continent.
BBS adequately monitors 204 species of North Americanbirds
that have a mean abundance of more than one bird per route and
a m l e size such that the d e w s of freedom (number of routes
on which the species is recorded minus the number of state-strata
units) ex&
14. For these species a change of 30/dyear over a
25-year period can be estimated This survey has its limitations,
however. Rat bbirds and birds of locally distributed habitats poorly
sampled by roads (e.g., mountain tops and wetlands) are
d e w l e d Birds that breed primarily north of the road system
in Canada (such as ptarmigans, jaegers, and shorebirds) or south
of the U.S. border (such as trogons, becards, and tropical raptors
and h m b i r d s ) and birds most active at night and at dusk
(rails, owls, goatsucken) are also undersampled Habitat changes
along roadsides may not be representative of the entire landscape,
so changes in some bird populations may be exaggerated on the
survey. On the other hand, effects of extensive clearcutting in the
west are underestimated if an amenity strip of udkhubed forest
is retained along roadsides.
Although survey procedures have not changed during the 26
years, analysis techniques and coverage are still being improved
(Sauer et al. unpubl. data, Peterjohn et al. 19-). From the beginning
routes have been grouped by physiogmphic regions within states
and provinces, and weighted according to the area they represent.
Formerly, only routes run in consecutive years were included in
the analyses, but now all routes covered at least two years by the
same observer are analyzed No two observers are identical in
knowledge and identification skills, necessitating use of observer
covariables in data analysis (Sauer et al. unpubl. data).
Even when observers are not changed, bias is likely for some
species. Observers' acuity increases with experience. This positive
bias, though slight, is detectible when comparing hrst and second
years a new observer runs a route. Increased familiarity with a route
may also result in higher counts, though this may be parhally offset
by a tendency for increasing d i m a n c e from mc over a period
of years. After about 50 years of age, an observer's ability to hear
distant high-pitched songs may decrease gradually. Hearing aids
can compensate in part for loss of hearing. Unfortunately, most
hearing aids are designed for assisting with comprehension of
speech rather than amphfyng higher frequencies. Even some
hearing aids with special "bird circuits" do not live up to claims to
a m ~ l l f ysounds higher than 6 kHz Thus birds with high-pitched
such as waxwings, gnatcatchers, several spruce forest
w*lers, and Gmshopper Sparmws, cannot be heard by most
observers over 60 years of age unless bids are close at hand. In
actual ~I-actice,
this is only a minor problem on BBS routes as the
propodon of observers unable to hear these few species is very
the
May W d l e r at 8-10 kHz and Blue-gray
Gmtatcher at 4-7 kHz are among the species reported as
"~mb
See Robbins et al. (1983) and Bondesen (1977) for
w e s of songs of ~ 0 1 t hAmerican birds.
WHY ARE SOME NEOTROPICAL
MIGRANTS DECLINING WHILE OTHERS
ARE NOT?
'
Although BBS detects population changes, it does not provide
m w n s that some species decline while others incmse. It is up to
biologists to provide hypotheses based on land-use changes on
breeding and wintering grounds, increased predation and nest
parasitism, weather conditions (especially e m m e conditions), and
environmental conditions during migration periods. If these
hypotheses are tested to determine whether future changes can be
predicted acmakly, it may be possible to explain some observed
changes. Effects of severe weather on breeding grounds are easy
to document (Robbins et al. 1986). On the o t l ~ hand,
r
land-use
changes usually occur so gmdually that drclmatic effects on bird
populations seldom occur on a continental scale. Nesting and
wintering habitats are being lost at the same time cowbirds are
expanding their ranges, and migration casualties are increasia
malung it diff~cultto determine from a breeding season survey
whether the greatest stress occurs in the U.S. and Canada or in the
tropics. Nearly all tropical American countries are losing min
forests, and no part of Lafin America has been identifled as showing
greater declines of migrants than another, based' on wintering
ranges. In the U.S., however, bird population declines are more
pronounced in eastern states than farther west.
In comparing species chamteristics in the first two paragaphs,
a few hypotheses begin to emerge. Among declining qxcies are
Wood Thrush and Kentucky Wdler, both of which need extensive
forest on bnding grounds and are restricted largely in winter to
mature rainforest in Latin America, where they feed on the ground
The Magnolia Whrbler, however, an increasing species, nests in
regenerating forest and winters commonly in citrus groves and other
agricultmd habitats as well as in primay forest Common birds of
grassland habitats may be in as much trouble as forest birds, though
they have received less attention Among species in trouble are the
Bobolink, Dickcissel and Vesper and Grasshopper Sparrows. We
are still in the early stages of isolating specific factors that control
populations of the many neotropical migrants, but we can make
several recommendations for reducing pressures on these birds.
EDUCATION AND MANAGEMENT
RECOMMENDATIONS
Stimulate Public Awareness of the Problem
We must stimulate public awareness of and grass roots
interest in the neotropical migrant problem throughout school
systems and through the medla, both in North America and in
the tropics. Literature and training programs are needed for land
managers, together with mandates for appropriate action The
Chesapeake Bay Critical Area legislation could be used as an
example of long-range land-use planning. There is also a
continuing need to promote natural history education and
ecotourism in tropical countries.
Improve Size, Shape, and Connectivity of Forest
Habitats
Much can be done to improve forest habitats for neotropical
migrants (Robbins 1979, 1991, Hanis 1984, Hunter 1990). For
example, land managers should keep forests in large blocks with
minimum edge, maximix the interior portion of the forest (that
portion farther than 100 meters from edge), minimize isolation
from other forests, and promote connecting wooded corridors.
Promote Diversity of Microhabitats
A high ,diversity of bird life within a forest requires a high
diversity of microhabitats with green vegetation at all heights.
A grazed or heavily browsed forest, for example, lacks the
critical ground layer of vegetation on which so many birds
depend for feeding and nesting. An even-aged forest lacks the
natural diversity of an uneven-aged forest.
Protect Against Exotics
Collaborate With Neighbors
Land owners can collaborate with neighbors and
community groups in mainfaining the maximum amount of
forest interior, in promoting wooded corridors, and pushing for
tax incentives for leaving land forested.
Reduce Mowing Frequency at Nesting Season
Peak
For field-nesting birds, managers can reduce mowing
frequency at the peak of nesting season and increase grassland
area that is more than 100 meters from other habitats. When
ec0,nomically feasible it helps to leave some fields fallow two
or more successive years to provide habitat for sparrows such
as Vesper, Lark, and Henslow's and for Dickcissels. As in
forested habitats, it helps to favor native vegetation and to
prevent overgrazing.
Provide Also for Early Successional Birds
For early successional species such as Prairie Warbler and
Yellow-breasted Chat that do not nest in narrow hedgerows,
extensive tracts of regenemting forest are needed. In some areas
this habitat is provided by succession following clearcuts; in
other places specific management may be needed.
Many native species of shrubs and trees cannot compete
with an invasion of exotic species.
Protect Wetland and Other Ground-nesting
Species
Cherish Old-growth Forest
Preservation of even small tracts of old-growth forest helps
promote diversity by providing habitats not otherwise present.
Where old-growth forest does not exist at present, it can be
created through long-range planning.
For wetland birds it helps to protect shores of ponds and
marshes from grazing so dense cover will be available for nest
sites. Access should be restricted during the peak of the nesting
season. In fact, for better breeding success of birds that nest on
or near the ground, household pets should be restmined during
at least the nesting season peak.
Cluster Snags Near the Forest Edge
WHY ARE WE CONCERNED?
Dead trees are important to many species of birds and
mammals for nesting sites, perches, and feeding opportunities.
However, dead trees are also used as perches by cowbirds
searching for songbird nests to parasitize. Many land
managers are retaining dead trees, often at some prescribed
rate of two or three per acre. We believe it would be more
appropriate to leave dead trees in clusters, preferably close
to forest edge, rather than scattered throughout the forest
interior, because neotropical migrants are most heavily
impacted by cowbird parasitism.
If long-term declines in neotropical mi*
are limited to
a relatively short list of species, why are we concerned?
We are concerned because changes detected to date may be
symptomatic of much greater changes still to come. The trend
toward more declines in the recent decade suggests a
deterioration of habitat conditions. There is still time to slow
declines and to make environmental changes to prevent these
and other species from disappearing from larger and larger areas
of our continent.
We are concerned over loss of species diversity for many
mans. Aside from moral considerations, there are compelling
practical reasons for concern, many of which are summarized
in Paul Kerlinger's paper in this symposium.
An intact ecosystem is of inestimable value for scientific
and educational reasons. Although we know many component
parts of our natural ecosystems, our knowledge of complex
internctions between various parts is rudimentary. Birds are
among the best understood members of the environment, yet we
are ignorant of the complex cycles of food abundance on which
they depend from week to week, and we are largely ignorant of
their role as seed dispersers and pollinators here in North
America and in the tropics.
By recognizing the problems, scientists, land use planners,
foresters, farmers, and other land managers can work together
to improve the landscape of this planet, not just for migratory
birds, but for all of us who live here.
LITERATURE CITED
Bondesen, P. 1977. North American bird songs-a world of
music. Scandinavian Science Press, Klampenborg, Denmark.
254 p.
Geissler, P.H., and J.R. Sauer. 1990. Topics in route-regression
analysis. Pp. 54-57 in J.R. Sauer and S. Droege, eds. Smey
designs and statistical methods for the estimation of avian
population trends. U.S. Fish and Wildlife Service Biological
Report 90(1).
Hagan, J.M., IU, T.L. Lloyd-Evans, and J.L. Atwood. 1992.
Long-term changes in migratory landbirds in the northeastern
United States: Evidence from migration capture data Pp.
115-130 in J.M.Hagan and D.W. Johnston, eds. Ecology and
conservation of Neotropical migrant landbirds. Manomet Bird
Observatory, Manomet, Massachusetts.
Harris, L.D. 1984. The fragmented forest: island biogeogmphy
theory and the preservation of biotic diversity. Univ. Chicago
Press, Chicago, Illinois.
Hunter, M.L., Jr. 1990. Wildlife, forests, and forestry: Principles
of managing forests for biological diversity. Prentice Hall,
Englewood Cliffs, New Jersey. 370 p.
Hussell, D.J.T., M.H. Mather, and P.H. Sinclair. 1992. Trends
in numbers of tropical- and temperate-wintering migrant
landbirds in migration at Long Point, Ontario. Pp. 101-114
in J.M.Hagan and D.W. Johnston, eds. Ecology and
conservation of Neotropical migrant landbirds. Manomet Bird
Observatory, Manomet, Massachusetts.
,
James, F.C., D.A. Wiedenfeld, and C.E. McCulloch 1992.
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southern highlands and increases in the lowlands. Pp. 43-56
in J.M.Hagan and D.W. Johnston, eds. Ecology and
conservation of Neotropical migrant landbirds. Manomet Bird
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Petejohn, B.G., J.R Sauer, and C.S. Robbins. 19--. The North
American Breeding Bird Survey and population trends of
Neotropical migrant birds. Pp.
in
eds., Ecology and management of
Neotropical migrant birds: a synthesis and review of critical
issues.
Robbins, C.S. 1979. Effect of forest fragmentation on bird
populations. Pp. 198-213 in R.M. DeGraaf and K.E. Evans,
eds. USDA Forest Service Technical Report NC-5 1.
Minneapolis, Minnesota.
Robbins, C.S. 1991. Managing suburban forest fragments for
birds. Pp. 253-264 in D.J. Decker, M.E. Kxasny, G.R. Goff,
C.R. Smith, and D.W. Gross, Challenges in the conservation
of biological resources: a practitioner's guide. Westview
Press, Boulder, Colorado.
Robbins, C.S., B. Bruun, and H.S. Zim. 1983. A guide to field
identification: birds of North America. Western Publishing
Co., Racine, Wisconsin. 360 p.
Robbins, C.S., D. Bystrak, and P.H. Geissler. 1986. The
Breeding Bird Survey: its f b t ffieen years, 1965-1979. U.S.
Fish and Wildlife Service Resource Publication 157. 196 p.
Sauer, J.R., and P.H. Geissler. 1990. Estimation of annual
indices from roadside surveys. Pp. 58-62 in J.R. Sauer and
S. Droege, eds. Survey designs and statistical methods for the
estimation of avian population trends. U.S. Fish and Wildlife
Service Biological Report 90(1).
Witham, J.W., and M.L. Hunter, Jr. 1992. Population trends of
Neotropical migrant landbirds in northern coastal New
~ n ~ l a ; l Pp.
d . 85-95 in J.M.Hagan and D.W. Johnston, eds.
Ecology and conservation of Neotropical migrant landbirds.
Manomet Bird Observatory, Manomet, Massachusetts.